Manufacturing with EDM | Advantages with EDM

US MANUFACTURERS GAIN AN EDGE

Lorincz, Jim

Small, delicate parts challenge processing

It's not a stretch to say that wire and sinker EDM technologies are cutting faster and putting a finer finish on products that are smaller, more delicate, and more complex than ever before.

Chief beneficiaries of these advances in EDM technology are US manufacturers in the mold and die, medical device, and electronics industries, among others searching for a competitive edge against their global competitors, principally from Asia. It's not too surprising, then, that many of the advances in technology are aimed at taking the EDM process deep into the realm of micromachining.

"To survive and compete US manufacturers must come up with new ideas and concepts to work with the very smallest parts," says Gisbert Ledvon, Charmilles (Lincolnshire, IL). Ledvon explains: "Electrodes are required for parts that are so small that you have to rely on your ability to cut the part perfectly, because they are difficult to measure."

Charmilles has adopted a two-pronged approach in developing its Roboform 350Î¼ MicroTEC technology for micromachining delicate, complicated parts. "First we have focused on developing application-driven technology that addresses the customer's requirements, for example, to produce a deep rib or produce a finish of a certain quality rather than to cut graphite into steel. Secondly, we have developed the MicroTEC generator to fine-tune power settings to match the complex details of smaller electrodes," Ledvon states.

The purpose of the MicroTEC discharge circuitry is to provide the EDM process the flexibility necessary to maximize material removal, while allowing a reduced electrode undersize of less than or equal to 0.002" (0.05 mm). "In this way," says Ledvon, "higher machining outputs are achieved, while maintaining the geometry of the electrodes due to an extremely low wear rate."

Machine accuracy is critical in small-cavity work or for positioning the electrode in multiple locations. Charmilles has replaced the former approach of using a drop tank and circulating dielectric flow around the table with a double thermostabilization system. The Robofbrm 350Âµ's main frame and X, Y, Z axes are enclosed and cooled by pulsed air, cooled in real time by the dielectric, which is circulated through holes drilled in the table. The result is that the whole machine tool is cooled by one chiller to one temperature.

"For micromachining, manufacturers need a lot more electrodes for all the small details of a mold. Having access to a lot of tools in the ATC is important, especially because tool changers for EDMs are fairly slow compared with those for milling machines," says Ledvon.

Charmilles has developed an ATC which is faster than previous models and has a storage capacity of 160 electrodes. The ATC has a double gripper that reduces toolchanging time from about 2 Â½ min to 55 sec. The ATC is a much less costly alternative to robotic tool changing, and it is integrated in the machine's control with collision protection.

A Renishaw optical transmission probe that's readily available can ensure positioning accuracy within a micron and is managed by the Roboform 350Âµ. Optical transmission of measurements allows dimensional inspection of machined cavities, as well as the taking of part references without having to remove it from the machine. All the measurements are sent back to the CNC, which then generates a complete inspection report.

"This is an important feature for medical and aerospace industries that require part-tracing information that the DPControl can generate automatically," says Ledvon. The DPControl interface offers a wide choice of machining strategies developed on the basis of the new MicroTEC settings, and generated automatically based on the data entered by the operator.

A lot of production work has shifted offshore, but there is still a great deal of work that's likely to remain, especially if the right technology is available. Jeff Kiszonas, EDM product line manager, Makino's Die/Mold Technologies (Auburn Hills, MI) explains: "There is still a lot of work staying here in the US in microminiature, medical, aerospace, and telecommunications applications, as well as prototyping and short-run mold or stamping-die work," he says, noting that "aerospace and DOD work tends to stay here."

"There is a movement, though not very strong yet at this point, to micromachining whether for miniature connector molds, miniature stamping dies, or miniature parts for medical implants, and telecommuncations products," says Kiszonas. "Drives for IPODs, and handheld video are getting smaller all the time, as are cell phones."

At IMTS, Makino introduced its EDAC1 ram EDM machine to the microconnector mold market for such products as cell phones and charger connectors. The EDAC1 is designed to deliver machining accuracy of Â±2Âµm and a corner radius of 5Âµm. "The ability to execute the tiny corner radii required by precision dies/molds for making connector parts and other components is increasingly in demand," says Kiszonas.

The EDAC1 ram EDM features an oil-cooled system that removes heat from the Z axis for improved depth accuracy. The SPG machining circuit provides surface finish of 0.5 Âµm R^sub y^ for carbide and 0.6Âµm R^sub y^ for SKD-61.

Introduced at IMTS 2004, the UPJ2 horizontal wire EDM has about 100 installations worldwide, and has been launched in North America for electronics, medical, telecommunications, and fiber-optic applications.

"The market for this type of machine is small but growing," says Kiszonas. "The machine can automatically thread tungsten wire to 0.0008" [0.020-mm] diam. In Japan, I recently saw one being used for plastic injection molds for gears that weigh about one millionth of a gram," he says.

Makino has introduced its HEAT, or High Energy Applied Technology, to enhance cutting speed and accuracy for its SP-43 and SP-64 wire EDM machines in production job shops, and aerospace and medical applications.

Using 0.010 or 0.012'' (0.25 or 0.30-mm) diam wire, HEAT can achieve part straightness of 0.0005'' TIR (0.013 mm) per side in one-pass machining and 0.0002'' TlR (0.005 mm) with two passes. HEAT technology can be used with both standard brass and coated wires (paraffin, nonparaffin, and high zinc).

Kiszonas explains: "HEAT greatly increases cutting speed in poor flushing conditions with detached upper and lower nozzles. Applications where the flushing nozzles are not sealed on the parts, or uneven part thicknesses, inherently cause problems with speed and wire breakage throughout the burn. Applications include medical hand tools and piece parts, as well as aerospace, electronics, fiberoptics parts and tooling, and contoured shapes, or where there are cross holes or counterbores."

Although the HEAT technology was developed specifically for tool steels, it has produced advantages in cutting stacked titanium plates. "We've seen some advantages to using HEAT for titanium because titanium burns like steel in a lot of applications. It has even been tried on Inconel with some good results, though there isn't much of an advantage for high-nickel chromium alloys used in aerospace."

MC Machinery Systems Inc. (Wood Dale, IL) has introduced two wire EDM machines, the Mitsubishi FA-PS and the Mitsubishi FA-VS, that advance the technology and capability of two existing machines, the FA-P and FA-V, the former noted for its accuracy, the latter for its speed.

* The FA-PS wire EDM is aimed at high-accuracy, small-wire fine finishing typical of the microtechnology world of miniaturization for connector tooling and small medical parts,

* The FA-VS wire EDM combines the speed of the FA-V machine with the accuracy and finish of the FA-S machine to reduce total part processing time through increased high-speed, rough-cut machining and improvements in finish machining.

"The FA-PS wire EDM is capable of running with wire from 0.002 to 0.012'' [0.05-0.30 mm] with start holes from 0.007'' [0.18-mm] diam with 0.002" [0.05-mm] wire," explains Greg Langenshorst, technical marketing manager, Mitsubishi EDM. "The previous FA-P machine ran a minimum wire size of 0.003'' [0.08 mm]. Most other standard machines, some requiring options, are capable of reaching just 0.004'' [0.10 mm]," he says.

The significance of this small-wire capability is that, in combination with the digital FS control, surface finishes to 3.0Î¼ in Ra, which is twice as good as previously attainable, are possible.

"The FA-PS brings a new level of customer work to the company," says Langenhorst. "Workpieces that require both high accuracy and fine finish, including carbide work, electronic connector tooling, or microchip-type tooling, as well as cutting tips of medical devices that need the finest burr-free surface finish, are leading candidates for the FA-PS," he says.

"Previously, for small-wire work you would have to use molybdenum or tungsten wire, but wire technology has advanced so that brass alloy wire to 0.002'' [0.05 mm] lends itself to faster cutting speeds, less expense, and is easier to use with machine technology," says Langenhorst.

The FA-PS isn't necessarily limited to using the smallest diam wire, either. Langenhorst remarks: "Machine movement has been smoothed out with AFC2, advanced friction control. Disturbances during high-precision feed and machine reversal are suppressed to produce a high level of roundness and pitch precision with a circularity error of 0.00005'' [0.0013 mm]. Precise positioning and Â±3Âµ machining accuracy are achieved."

Langenhorst compares the new FA-VS wire EDM machine to a race car, combining the high speed of the V machine with the accuracy and surface finish of the S machine. "The high-speed V500 power supply produces machining speeds to 500 mmVmin and improved machining performance for difficult-to-cut materials, like titanium alloy or heat-resistant high-nickel alloys." For piecepart production machining the FA-VS can automatically thread 0.36-mm-diam V wire.

"Some of the key features in the V500 power supply lend it to cutting PCD or CBN and graphite like POCO EDM 200, a very popular grade of graphite for sinker use," says Langenhorst: "It's a less-expensive grade than, say, EDM 3, being a coarser-grain material with high tar levels. It has a tendency to heat-fracture and stress-crack more from high spark intensity. The V500 sails right through it with no signs of micro-fracturing. It also works better in high-temperature materials like Hastelloy, Waspaloy, and Inconel, among others."

"Total part processing time is improved by 20% or more by the FA-VS through increased high-speed, roughcut machining and improvements in finish machining. Surface finish to 9-10Âµ in R^sub a^ is twice as good as the previous FA-V model. High-speed, high-accuracy machining with 3Âµm straightness is possible with a minimum number of finish cuts," Langenhorst says.

The FA-VS also benefits from the Corner Master 2 control that attains a uniform electrical discharge gap regardless of the corner shape. The control improves radial shape error and total part accuracy. The Power Master PM4 control enables full automation from rough to finish machining, allowing stepped shape capabilities for processing multiple parts with different shapes and thickness to be machined without setting specific electrical conditions.

Jim Lorincz

Senior Editor